German embryologist and philosopher who is best known as one of the last advocates of vitalism, the theory that life is directed by a vital principle and cannot be explained solely in terms of chemical and physical processes. Nevertheless, he also made several important discoveries in embryology, although these have tended to be overlooked because of his mistaken belief in vitalism.

Driesch was born on 28 October 1867 in Bad Kreuznach, the son of a prosperous gold merchant. He studied zoology, chemistry, and physics at the universities of Hamburg, Freiburg, Munich, and Jena, obtaining his doctorate from Jena - where he studied under Ernst Haeckel - in 1887. Coming from an affluent family, Driesch had no need of paid employment and he spent the next 22 years privately pursuing his embryological studies. After obtaining his doctorate, he travelled extensively in Europe and the Far East, spending nine years from 1891 working at the International Zoological Station in Naples. In 1899 he married Margarete Reifferscheidt; they later had two children, both of whom became musicians. Eventually Driesch settled in Heidelberg, and in 1909 he was appointed lecturer in philosophy at the university there, becoming professor of philosophy in 1911. Subsequently he was professor of philosophy at Cologne University 1920-21 and at Leipzig University 1921-35, when he was forced to retire by the Nazi regime. Driesch died in Leipzig on 16 April 1941.

In 1891 Driesch, experimenting with sea urchin eggs, discovered that when the two blastomeres of the two-cell stage of development are separated, each half is able to develop into a pluteus (a later larval stage) that is completely whole and normal, although of smaller than average size. Similarly he found that small, whole individuals can be obtained by separating the four cells of the four-cell stage of development. From these findings he concluded that the fate of a cell is not determined in the early developmental stages. Later, other workers discovered the same phenomenon in the early developmental stages of hydroids, most vertebrates, and certain insects. (It should be noted, however, that not all animal eggs behave in this way; for example, separation of the early embryonic cells of annelids, molluscs, and ascidians results in incomplete embryos.) Subsequently Driesch produced an oversized larva by fusing two normal embryos, and in 1896 he was the first to demonstrate embryonic induction when he displaced the skeleton-forming cells of sea urchin larvae and observed that they returned to their original positions. These findings provided a great impetus to embryological research but Driesch himself - unable to explain his results in mechanistic terms (principally because at that time very little was known about biochemistry)- came to believe that living activities, especially development, were controlled by an indefinable vital principle, which he called entelechy.

After his appointment as a lecturer in 1909, Driesch abandoned scientific research and devoted the rest of his life to philosophy.